1. Field of the Invention
The present invention relates to a liquid developer for use in electrophotographic copying apparatus, conventional printers, facsimile apparatus, and ink-jet printers.
The present invention also relates to a method of producing the above-mentioned liquid developer and an image formation method using the same.
2. Discussion of Background
Developers for use in electrophotography are roughly classified into two groups, that is, a dry developer and a wet developer (or liquid developer). The liquid developer is capable of producing clearer images than the dry developer due to the use of toner particles with a smaller particle size therein. Because of this advantage, the merits of the liquid developer have been discovered in recent years.
In general, a liquid developer for use in electrophotography is prepared by dispersing a toner comprising a coloring agent and a binder agent as the main components, optionally with the addition of a charge controlling agent thereto, in a carrier liquid.
The coloring agent comprises, for example, carbon black, an organic pigment or dye stuff. Examples of binder agents are natural or synthetic resins such as acrylic resin, phenol-modified alkyd resin, rosin and synthetic rubber. Examples of charge controlling agents are lecithin, metallic soap, linseed oil and higher fatty acids. The carrier liquid comprises as the main component a highly-insulating solvent with a low dielectric constant, such as a petroleum-based aliphatic hydrocarbon solvent.
In an electrophotographic process using such a liquid developer, latent electrostatic images are formed on a photoconductor and developed with a liquid developer prepared by diluting a concentrated liquid toner with a solid content of 5 to 90 wt. % with a carrier liquid having high insulating properties. More specifically, for example, in an electrophotographic copying machine, a bottle for the concentrated liquid developer and a bottle for the carrier liquid are disposed. As the concentrated liquid toner and the carrier liquid are consumed in the course of making copies, the concentrated liquid toner and the carrier liquid are replenished in accordance with the detection of the consumption of the respective components.
After development of latent electrostatic images formed on the photoconductor with the liquid developer, an excess of the liquid developer is removed from the surface of the photoconductor by a corona discharger or a squeeze roller which is rotated in the opposite direction to the rotating direction of the photoconductor, but out of contact with the photoconductor. The toner images remaining on the photoconductor are then transferred to a transfer sheet, and fixed thereto.
In the case where the excess of the liquid developer is removed from the photoconductor, using the squeeze roller, when the viscosity of the liquid developer is excessively high, the developer cannot be sufficiently removed from the surface of the photoconductor by the squeeze roller, and too much an amount of the liquid developer to develop clear images stays on the photoconductor, while when the viscosity of the liquid developer is excessively low, the developer is excessively removed from the surface of the photoconductor by the squeeze roller, and too small an amount of the liquid developer to develop clear images is left on the photoconductor. The result is that in either case, images with uniform and sufficient density cannot be obtained, in particular, in solid image areas.
The toner images formed on the photoconductor are transferred to a transfer sheet such as plain paper. The transfer ratio of the toner particles in the liquid toner from the photoconductor to the transfer sheet varies depending on the properties of the transfer sheet, such as surface smoothness, oil absorption and thickness, but is generally in the range of 50 to 100%. The toner transfer ratio is defined by the ratio of the weight of the toner particles transferred to the transfer sheet to that of the toner particles deposited on the photoconductor before toner image transfer.
When paper with low surface smoothness such as a bond paper is used for image transfer, the toner transfer ratio is as low as 50%. In such a case, it is necessary to remove the remaining liquid toner from the surface of the photoconductor, using cleaning means such as a cleaning blade after image transfer. If the liquid toner remains on the photoconductor even after the cleaning operation, and is repeatedly subjected to the charging, exposure and development in the course of the electrophotographic process, the liquid toner is formed into a toner film on the photoconductor, which is referred to as "toner filming phenomenon", and the thus formed toner film causes the formation of abnormal images, such as smeared toner images transferred to a transfer sheet.
The above-mentioned toner filming phenomenon has attracted special attention in recent years as being a very serious problem when color toners, in particular cyan-color toners, are used. Many trials have not yet been made for solving this filming problem for color toners, and the problem has not yet been solved.
In Japanese Laid-Open Patent Application 60-179750, there is proposed a toner comprising an acid amide compound in light of the toner filming problem. When the toner was subjected to a continuous copying operation by a conventional development method, the occurrence of the filming phenomenon was observed after making about 600 to 1,500 copies. Thus, high quality image cannot be produced for an extended period of time by a conventional development method.
In Japanese Laid-Open Patent Application 49-071943, there is proposed a developer comprising as a charge controlling agent a higher alkylamine compound or a quaternary ammonium salt compound in order to improve the tone reproduction of images. However, the tone reproduction becomes poor when a concentrated toner for use in the liquid developer is stored for a long period of time.
In Japanese Laid-Open Patent Applications 51-024244 and 58-052652, there are disclosed liquid toners. These liquid toners comprise a higher alcohol and show the same tendency as mentioned above, that is, the tendency of the tone reproduction becoming poor when a concentrated toner for use in the liquid developer is stored for a long period of time.
Furthermore, in the course of the development which is carried out, using the liquid toner or developer, the toner composition is not necessarily uniformly attracted to the latent electrostatic images by electrophoresis. Finely-divided components of the toner, such as finely-divided components of a resin, a dispersant and a pigment are gradually dissolved into the carrier liquid in the course of the repeatedly carried out development, and such finely-divided components are built up in the carrier liquid. As a result, the viscosity of the liquid developer is gradually excessively increased in the course of the repeatedly carried out development, and as mentioned previously, when the viscosity of the liquid developer becomes excessively high, the developer cannot be sufficiently removed from the surface of the photoconductor by the squeeze roller, and too much an amount of the liquid developer to develop clear images remains on the photoconductor, causing the formation of abnormal toner images.